AbstractVHfragments were prepared from several mouse IgM molecules by cyanylation. In all cases VHfragments were purified to homogeneity by using Ig light chain affinity columns. Several different anti‐VHantisera were prepared in rabbits and the specificities of these antibodies were studied. Two patterns of cross‐reactivities were observed: (a) some anti‐VHantibodies reacted only with closely related VHmolecules,e.g., anti‐VHHPC52 anti bodies reacted only with VHof phosphorylcholine‐binding myeloma or hybridoma protesin, and concrodantly, stained about 4% of mouse spleen B cells; (b) on the other hand, antisera‐like anti‐VH104E and 8916 antibodies were very cross‐reactive. Binding assays showed that both of these anti‐VHantibodies reacted with 50–60% of mouse immunoglobulins, and thus the pool of these antibodies reacted with about 95% of mouse VHregions. Concordantly, anti‐VH104E antibodies stained in the fluorescence‐activated cell sorter (FACS) analysis more than 50% of mouse spleen B cells.Cross‐reactive anti‐VHantibodies (“anti‐framework”) did not stain T cells nor did they immunoprecipitate VH‐like molecule